Frequently JAR/JAD mismatches occur to due caching. Sprint's servers will cache a copy of your files and if you are making frequent changes then there is a good chance you are downloading a version older than you expect.

To get around the caching issue, append a random number on to the JAR URL and JAD URL. For example:
[code:1:438562f9a4]MIDlet-Jar-URL: http://apgap.com/MyMidlet.jar?r=848239484[/code:1:438562f9a4]
Make sure you change the number every time you change the JAR. You need to do the same thing with the JAD. For example, this MIDlet would be downloaded like this:
[code:1:438562f9a4]http://apgap.com/MyMidlet.jad?r=4635664[/code:1:438562f9a4]